from abc import ABCMeta, abstractmethod

class PCBManager(object):
    
    """abstract class, demand protocol to subclasses"""
    __metaclass__ = ABCMeta
    
    @abstractmethod
    def next(self):
        raise NotImplemented()

    @abstractmethod
    def add(self,_pcb):
        raise NotImplemented()
    
    @abstractmethod
    def has_next(self):
        raise NotImplemented()